feat: hex init nu (#3330)

- **feat(hex): add nushell support for `atuin hex init`**
- **docs(hex): add nushell hex setup instructions**

This adds setup for nushell. It breaks the pattern of calling `eval
$(atuin init)` on behalf of the user because nushell simply cannot do
this. I tried to source the atuin.nu file and add the preamble to it,
but it is part of the atuin package, so it made things too difficult.

I think settling for separate init is ok.

Partially addresses #3329.

Please see #3323 as well.

I was able to get it working and I am using these changes in my
dotfiles:

https://github.com/xav-ie/dots/compare/b1a8cf96b58f802396ac5103f0925e1434fc473c...696dbf31008395587353e3071f5296c459685a17

Basically, I just do as the new docs say and make sure to add hex init
before regular init and source it.

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